Currently, a liquid crystal element which is the display panel using liquid crystal material has been broadly used for plane display elements. The liquid crystal elements of this type have been used in many areas, and examples of application products of such liquid crystal elements include smartphones, portable game machines, tablets, Notebook type PCs, and TV receivers.
However, a plane display element in practical use has two faces (hereinafter referred to as “front side” and “back side” for explanation purpose). An image is displayed only on the front side, and display operation cannot be performed on the back side (see FIG. 8). FIG. 8 illustrates the case of using a liquid crystal element. In the case of the liquid crystal element, a backlight 802 is provided in the back of a liquid crystal element 801, and a display direction (a light passing direction 804) is only one direction of the both faces so that the liquid crystal element 801 is irradiated with light of the backlight 802 to perform the display operation.
Since the display element emits light only in one direction as described above, application products using such a display element are designed on the condition that light is emitted in one direction. Of products classified as portable information terminals, such as smartphones and tablets, an information terminal (hereinafter referred to as a “slate-type terminal,” and see FIG. 2A) configured such that a display element extends across the substantially entirety of one side of the product does not include a keyboard as a character input device in a typical case, and therefore, is not suitable for the purpose including an input operation. There is another design employed typically for transportable Notebook type PCs and allowing an input operation mainly using a keyboard. An information terminal (hereinafter referred to as a “clamshell-type terminal,” and see FIG. 2B) having this design is configured so as to be folded with a display and a body facing each other in a normal state and to be used with the display being opened when necessary. In the case of using the design of the clamshell-type terminal, the display has to be opened before use. For this reason, the clamshell-type terminal is not suitable for the purpose of use in any attitudes as in the slate-type terminal, and is intended basically for use in a seated attitude.
As described above, although the slate-type and clamshell-type terminals have both merits and demerits. The products having combined merits of both type terminals have been practically applied. For example, there is a product employing the structure in which a keyboard as an input device is disposed at a cover of a slate-type terminal. However, in the case of using the product with such a structure, a heavier body is unstable in an upright attitude, and there is a probability that troubles such as falling over during use are caused. For these reasons, this product is not suitable for, e.g., the operation on the laps.
On the other hand, when an attempt is made to use the clamshell-type terminal as in use of the slate-type terminal, it is necessary to rotate the display 180 degrees by some kind of means. In order to provide such a configuration, the following structures have been proposed:
1) The structure in which two hinges are provided, which include a hinge for bringing a terminal from a closed state to an open state and a hinge for rotating a display 180 degrees;
2) The structure in which two hinges are provided so as to bring a terminal from a closed state to an open state and to open the terminal to cause a display to rotate to the back side of a body; and
3) The structure in which hinges are provided respectively at ends of a body and a middle portion of a display to rotate the display.
In any of the above-described cases, at least one hinge whose strength is maintained and which includes the complicated mechanism for transmission of electrical signals is additionally provided. Since it is undeniable that an unreasonable configuration is made as compared to a simple clamshell-type portable
terminal, there are concerns over lowering of strength and an increase in a failure probability. Moreover, a special hinges with a complicated structure is used, resulting in a cost increase.
These problems are caused because of the following reason. Since the light passing direction of the display element used for the display is only the direction from the front side, the mechanism for rotating the display is essential when a combination terminal of slate-type and clamshell-type terminals are used.
In order to solve these problems, the clamshell-type terminal is used as a basic configuration, and the outputted light is emitted from the display not only from the front side but also from the back side. With this configuration, even in the state in which the display is stacked on the body (a slate state) or the state in which the input device, such as a keyboard, provided at the body is used with the display being opened (a clamshell state), a user can view a display image.
Patent Literature (PTL) 1 has, as the above-described liquid crystal element which can be viewed from any of the front and back sides of the display element, disclosed a both-sided liquid crystal element having the structure in which a backlight is sandwiched between liquid crystal elements.
FIG. 9 illustrates a basic configuration diagram in the case of emitting light not only from the front side but also from the back side. FIG. 9 illustrates a basic structure in the case of using the liquid crystal elements, and the structure is made such that a backlight 902 of emitting light to both sides is disposed in the middle and sandwiched between liquid crystal elements 901a, 901b. With such a structure, the light of the backlight 902 disposed in the middle can transmit through the first liquid crystal element 901a and the second liquid crystal element 901b, and the operation to pass the light to both sides (a light passing direction 904) can be performed with the single backlight shared by the liquid crystal elements 901a, 901b. 
An element capable of switching between a light transmission state and a light reflection state has been proposed as another related art. A light control mirror (hereinafter referred to as a “light control element”) has been known as such an element capable of electrically switching between the light transmission state and the light reflection state.
A basic structure of this light control element is, e.g., the structure illustrated in FIG. 4 of Non Patent literature (NPL) 1. The principle of the light control element has been found by Professor Griessen et al. of VU University Amsterdam as described in NPL 3 and PTL 2. According to this principle of operation, a metal thin film of rare earth such as yttrium (Y) and lanthanum (La) is bonded to hydrogen, thereby reaching a transparent state. After the hydrogen is removed, the metal thin film is changed into a state (a mirror surface state) with metal luster. In injection and removal of the hydrogen, palladium functions as a catalyst, and therefore, a slightly-thin palladium layer is formed on the surface of the rare-earth metal thin film.
Although the light control element uses hydrogen gas to switch between the transparent state and the mirror surface state, it is necessary to, without using hydrogen in gaseous state, inject hydrogen to the metal layer and remove the hydrogen from the metal layer in order to use the light control element as an electronic device. Moreover, since rare-earth metal has poor efficiency, development has been later made on another metal film.
The structure disclosed in NPL 1 is an example where an all-solid light control element is realized by employing the structure capable of inserting hydrogen atoms into a magnesium-nickel alloy layer and removing the hydrogen atoms from the magnesium-nickel alloy layer without using gas. NPL 1 describes that magnesium-titanium alloy or magnesium-niobium alloy can be used for the metal layer. This light control element requires about several seconds to change the state thereof. However, since such a state changes with a direct current of about 5 [V], the light control element can operate even under the conditions where high voltage cannot be supplied, e.g., in the case of a portable information device. In light reflection direction change which is one of the objectives of the present invention, a state change time of about several seconds does not cause any problems.
The above-described conventional example where the all-solid light control element is applied to the display device includes an example where an all-solid light control element is applied to a semi-transmissive liquid crystal element as described in PTL 3. PTL 3 describes the element configured as a reflective liquid crystal element. Thus, the side of the light control element serving as a mirror in a reflection mode faces an observer. PTL 6 describes, as another example, an example where a light control element is used to switch displaying between displaying in one direction and displaying in both directions. In the structure of PTL 6, configuration is made such that a liquid crystal element is disposed in the middle and that pairs of a light guide plate (a member for guiding light of LEDs provided at end portions to emit light in a planar pattern) and the light control element is arranged with the liquid crystal element being interposed between the pairs of the light guide plate and the light control element. In this configuration, light can be guided in any of both directions of a display element. Moreover, PTL 4 discloses, for the purpose of different application, a device configured to switch, by a light control element disposed as a cover glass of a display element, between a state for information displaying and a state for use as a mirror allowing an observer to view oneself. According to PTL 4, the light control element is disposed on the surface to block displaying, thereby providing the function of the mirror. Further, although not applied to a display device, PTL 5 describes a conventional example where a light control element is, as a screen for projection displaying, used with a scattering reflection-transmission configuration. PTL 5 describes combination of a scattering-transmission type light control element and a reflection-transmission type light control element.